This invention relates to circuits and methods for controlling electrical power systems and, more particularly, to such circuits and methods which control the paralleling of alternating current power sources such that the paralleling connection is made when the requirements of synchronism are met.
When alternating current power sources, such as generators or inverters, are to be operated in parallel, the sources must be close to synchronism with each other when they are connected together, or when an incoming power source is connected to a line or bus to which other power sources are already connected. This means that the voltage, frequency and phase angle of the incoming power source and of the line must be close to the same values, within predetermined limits, in order to prevent excessive system transients, and so that the incoming source can be pulled into synchronism with the line.
When AC power sources are provided with control means or regulators which hold the frequency and voltage of the outputs close to desired values, paralleling can be accomplished by providing sensing means for sensing the frequency and phase angle differences between the incoming source and the energized line. The output of the sensing means then controls a circuit breaker of the incoming source to cause the breaker to close at the correct instant when the incoming source is sufficiently close to synchronism with the energized line.
Auto paralleling circuits are designed to monitor the phase angle and voltage synchronization of electrical systems which are to be paralleled. Voltage matching of the power sources is usually assumed to be controlled adequately by individual source voltage regulators. The phase angle and slip frequency must be monitored to be within specified limits before the paralleling contactor can be closed.
Numerous examples of automatic paralleling circuits can be found in the prior art. Such circuits are disclosed in U.S. Pat. Nos. 2,862,111; 3,069,556; 3,210,556; 3,444,387; and 3,588,519.